A method and apparatus for transmitting data from a transmitter device to one or more receiver devices, each of which is connected to the transmitter device via a respective wire connection, the transmitter device being operable to transmit signals onto the wire connections over different channels, the method comprising, for each channel: determining a value of a first function which is a function of a power mask for transmitting the data over the channels; determining a value of a second function which is a function of an upper bound for a channel capacity of the channel; determining a value of a third function, the third function being a function of the first and second functions; using the determined value of the third function, determining a transmission power for that channel; and transmitting, by the transmitter device, data over that channel at the transmission power determined for that channel.